In recent years, color liquid crystal display devices have been widely used in various fields as monitors of portable notebook personal computers, personal computers, and so forth, or as display portions of liquid crystal televisions, video-combined liquid crystal televisions, portable telephones, portable information terminals, and so forth. Further, following increase in information processing amount, diversification of needs, multimedia adaptation, and so forth, increase in screen size and fineness of the liquid crystal display devices has been actively developed.
The liquid crystal display device basically comprises a backlight portion and a liquid crystal display element portion. For the backlight portion, there is an under-light type wherein a light source is disposed directly under the liquid crystal display element portion, or an edge-light type wherein a light source is disposed facing a side end surface of a light guide. In terms of making compact the liquid crystal display device, the edge-light type has often been used.
In recent years, for a display device with a relatively small screen size wherein a viewing direction range is relatively narrow, such as a liquid crystal display device used as a display portion of, for example, a portable telephone, use has been made of, in terms of reducing power consumption, one, as a backlight portion of the edge-light type, that reduces a spread angle of light emitted from a screen as much as possible to emit light in a concentrated manner in a required angle range for effectively utilizing the quantity of light emitted from a primary light source.
In the display device wherein the viewing direction range is limited as described above, as a light source device that emits light in a concentrated manner in a relatively narrow range for increasing the utilization efficiency of the quantity of light of a primary light source to thereby reduce the power consumption, the present applicant has proposed in JP(A)-2001-143515 to use a prism sheet having prism-formed surfaces on both sides thereof so as to dispose it adjacent to a light outgoing surface of a light guide. In this double-sided prism sheet, a light entrance surface being one surface and a light exit surface being the other surface are respectively formed with a plurality of mutually parallel elongated prisms, wherein the directions of the elongated prisms are matched with each other on the light entrance surface and the light exit surface and further the elongated prisms are disposed at corresponding positions on both surfaces. With this configuration, light having a peak of outgoing light in a direction inclined relative to the light outgoing surface of the light guide and exiting from the light outgoing surface so as to be distributed in a proper angle range is incident on one-side prism surfaces at the light entrance surface of the prism sheet, then is internally reflected by the other-side prism surfaces, and is further subjected to a refraction action by the elongated prisms at the light exit surface, so that the light is emitted in a concentrated manner in a relatively narrow, required direction range.
According to this light source device, the concentrated light emission in the narrow angle range is enabled. However, in the prism sheet used as a light deflector, the plurality of mutually parallel elongated prisms are required such that the directions of the elongated prisms are matched with each other on the light entrance surface and the light exit surface, and further, the elongated prisms are disposed at the corresponding positions on both surfaces. This formation is complicated.
Further, there has also been a problem that since the light emitted in a concentrated manner in such a narrow angle range causes strong glare, the quality as the backlight is deteriorated. Moreover, although such a light source device having the narrow viewing angle raises no problem in case of a small-size backlight, there has also been a problem that, as a backlight for a liquid crystal display device of 4 inches or greater, particularly as a backlight used in a liquid crystal display device of about 12 to 15 inches for a notebook personal computer, its viewing angle is so narrow, resulting in poor visibility.
It is therefore an object of the present invention to provide a light source device excellent in light utilization efficiency of a primary light source, and also excellent in viewing angle characteristic and quality for image forming illumination without sacrificing a high luminance characteristic.